Remove redundant echo.mojom, verify non-nullables in echo_service.mojom

Remove redundant echo.mojom, verify non-nullables in echo_service.mojom

While looking at mojom files I'm nominally the owner of, I realized
that one had become redundant.  So, I just deleted it and checked the
other to make sure it had the right 'nullability' on the method args.

Also moved around code to build against the de-duped mojom file.

BUG=407673
TEST=build with gyp and gn

Committed: https://crrev.com/0e989ee10367671221add030b0cb55e42311faf1
Cr-Commit-Position: refs/heads/master@{#292054}

[Chris Masone, 2014-08-26 20:40:19]

cmasone@chromium.org changed reviewers:
+ yzshen@chromium.org

[Chris Masone, 2014-08-26 20:40:19]

PTAL

[yzshen1, 2014-08-26 23:24:30]

LGTM Thanks!

[Chris Masone, 2014-08-26 23:29:59]

The CQ bit was checked by cmasone@chromium.org

[commit-bot: I haz the power, 2014-08-26 23:33:21]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/cmasone@chromium.org/503363004/1

[commit-bot: I haz the power, 2014-08-27 00:32:20]

Committed patchset #1 (1) as cdc160da131724198be79a69e4b2ccabbb533589

[commit-bot: I haz the power, 2014-09-10 02:48:13]

Patchset 1 (id:??) landed as
https://crrev.com/0e989ee10367671221add030b0cb55e42311faf1
Cr-Commit-Position: refs/heads/master@{#292054}